1. Field of the Invention
The invention relates generally to a press forming method and a press forming device. More specifically, the invention relates to a press forming method for press-forming a panel part made of aluminum base alloy, and a press forming device that is used to perform the press forming method.
2. Description of the Related Art
In recent years, the needs for vehicles having lighter weights have been growing. In light of such situation, more expectations have been placed on increases in the use of panel parts (hood panels, fender panels, etc.) made of aluminum base alloy. A panel material made of aluminum base alloy (a panel material made of aluminum base alloy will be simply referred to as an “aluminum material” in the following description, although it will be sometimes referred simply to as a “panel material”) has a yield strength and a tensile strength that are at substantially the same levels as those of commonly used mild steels. Meanwhile, an elongation and a strain value (r value) of the aluminum material are lower than those of the commonly used mild steels. Accordingly, the aluminum material and the mild steels are greatly different from each other in the manner in which they respond to the press forming. A lot of attention is required when the aluminum material is pressed into shapes. Generally, as shown in FIG. 9, when an aluminum material 11 is pressed into shapes to form a fender panel 1, a press direction and a die face, which are used during the press forming, are set using a plain face Q including three points P1, P2, and P3 of the fender panel 1, as a reference face. In this case, the limit to which the aluminum material 11 can be drawn is approximately 200 mm (i.e., the aluminum material 11 can be drawn by 200 mm at the maximum). In addition, changes in the cross section of the aluminum material 1, which occur due to the press forming, need to be limited to prevent crinkling and cracking. As a result, the design of the fender panel 1 is significantly limited.
Recently, the variety of design of vehicle bodies has been increasing. Due to such increases, a dimension of the fender panel 1, produced by the press forming, on a given plane face when viewed from the top of the fender panel 1 (hereinafter, such dimension will be referred to as a “plane dimension”) may be equal to or greater than 500 mm. When the aluminum material 11 is pressed into shapes to form the fender panel 1 having a great plane dimension, a maximum drawing depth S1, at which the aluminum material 11 is drawn by the maximum amount (see FIG. 9), significantly exceeds the limit to which the aluminum material 11 can be drawn. As a result, cracking occurs. Also, when the aluminum material 11 is pressed into shapes to form the fender panel 1 having a great plane dimension, the press forming is started from the center of a portion at which the aluminum material 11 is drawn by the maximum amount. Accordingly, as shown in FIG. 10, the portion of the aluminum material 11, which will be the fender panel 1, is not pressed uniformly. In addition, a punch (a lower die) and the aluminum material 11 contact each other at a point. As a result, crinkling occurs in a product. Clinking and cracking can be prevented to some extent by, for example, appropriately arranging the die face and an addendum shape portion and adjusting the amount of flow of the material caused by the press forming. However, when the cross section of a bent portion greatly changes, for example, when the fender panel 1 having a great plane dimension is formed from the aluminum material 11, occurrence of clinking and cracking cannot be prevented.
Japanese Patent Application Publication No. JP-A-2004-188445 describes a method for pressing an aluminum base alloy plate into shapes. According to this method, an end portion of the aluminum base alloy plate is bent in advance to form a bent portion. This bending process is performed so that, a portion, that is included in the end portion of the aluminum base alloy plate, the end portion being formed into the bent portion, and that is in contact with a binder surface (die face) during the press forming, is roughly conformed to a surface of a blank holder during the press forming. Then, the aluminum base alloy plate is pressed into shapes, while blank holding is performed on the aluminum base alloy plate including the portion of the bent portion, which is in contact with the binder surface during the press forming. However, the described method does not make it possible to uniform the drawing depth if the cross section of the bent portion greatly changes, for example, if the fender panel 1 having a great plane dimension is formed from the aluminum base alloy plate. As a result, the portion of the aluminum base alloy plate 11, which will be the fender panel 1, is not pressed uniformly, causing crinkling in a product. If superplastic forming (blow forming) is adopted here, the fender panel 1 made of aluminum base alloy, which has a great plane dimension and high-quality, can be produced. However, aluminum material for superplastic forming is more expensive than aluminum material for press forming. As a result, performing superplastic forming increases production cost.
In addition, the cycle time of the superplastic forming is longer than that of the press forming. Therefore, performing the superplastic forming significantly reduces the production efficiency (for example, although the cycle time when the fender panel 1 made of aluminum base alloy is produced by press forming is 7.5 seconds per one piece, the cycle time when the fender panel 1 made of aluminum base alloy is produced by superplastic forming is 5 minutes per one piece). Alternatively, the fender panel 1 may be produced by brazing a plain face portion and a side face portion to each other, which are individually formed by press forming. However, this method does not provide any of high quality, high production efficiency (the cycle time when the fender panel 1 made of aluminum base alloy is produced by this method is three hours per one piece) and high cost performance (multiple types of dies are required).